OFDMA
Orthogonal frequency-division multiplexing (OFDM) is a modulation technique used at the physical layer (PHY) of a number of wireless networks, e.g., networks designed according to the well known IEEE 802.11a/g and IEEE 802.16/16e standards. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access scheme based on OFDM. In OFDMA, separate sets of orthogonal tones (subchannels) and time slots are allocated to multiple transceivers (users or mobile stations) so that the transceivers can communicate concurrently. OFDMA is widely adopted in many next generation cellular networks such as 3GPP Long Term Evolution (LTE) and IEEE 802.16m due to its effectiveness and flexibility in radio resource allocation.
Interference in a Multi-Cell OFDMA Network
The fundamental challenge in wireless communications is the inequality between the scarce spectrum that is available, and the vast area to be covered and large number of users to be served. In other words, the same frequency spectrum must be reused in multiple geographical areas or cells. This will inevitably incur inter-cell interference (ICI), when users or mobile stations (MSs) in adjacent cells use the same spectrum. In fact, ICI is the predominant performance-limiting factor for wireless cellular networks.
In order to maximize the spectral efficiency, frequency reuse factor of one is used in OFDMA cell deployment, i.e., the same spectrum is reused in each and every cell. Unfortunately, this high spectrum efficiency is also accompanied by high detrimental ICI. Therefore, a good ICI management scheme on top of OFDMA is needed to leverage the multiple access benefits of OFDMA technology.
Base Station Cooperation (BSC)
Along with OFDMA another widely adopted technology for existing and next generation cellular networks is Multiple Input Multiple Output (MIMO). The technology aims to increase spectral efficiency is the use of multiple antennas at both the transmitter and the receiver. The multiple antenna elements allow for multiple independent spatial channels between the transmitting and receiving terminals which can be used to either increase the spectral efficiency (via spatial multiplexing) or improve the reliability (via antenna beamforming) of communication.
Base station cooperation (BSC) is an extension of the MIMO concept to geographically separated base stations. The technique allows multiple BSs to transmit signals to multiple MSs concurrently sharing the same resource (i.e., time and frequency). It utilizes the SDMA technique for BSs to send signals to MSs cooperatively and is specifically used in cell-edge MSs that are within the transmission ranges of multiple BSs. Effectively the channel among the cooperating base stations and the users can be view as a MIMO channel, albeit with a much larger number of antennas then between a single base station and user. Thanks to cooperation, the interfering signal becomes part of the useful signal. Thus, BSC has two advantages: provision of spatial diversity and ICI reduction.